The present invention relates to fluorescence-based devices for detecting microbial growth from test samples.
It is necessary to test various industrial substances, such as food, pharmaceuticals, cosmetics and water, for microbial contamination. One area of biological testing of food, dairy, pharmaceutical, cosmetic and related types of products involves the estimation of total numbers of bacteria, yeasts and molds, as well as concentrations of specific groups of organisms within the material. One widely used method is known as the “Standard Plate Count” method and involves culturing a diluted sample of the product in an agar growth medium. The plates containing the sample and the growth medium are incubated (e.g., 32° C.-40° C.) for 24 hours to 5 days, depending upon the assay. After incubation, colonies of microorganisms which have grown in the agar are counted.
Optical methods have been successfully used to classify microorganisms in clinical samples (e.g., PASCO by Difco, Detroit, Mich.). Although it would be desirable to utilize a colorimetric method, or any other optical method, for detecting microbial growth in industrial samples, the solid substances of the test samples disposed in an aqueous media usually cause optical interference for a detection system. More specifically, when solid substances are disposed in a media to allow for culturing microorganisms, the colorimetric detection system must pass light either through or reflect light from the media containing the solid substance. In most of the cases, the solid substances interfere with the spectral characteristics of the media, yielding a poor signal-to-noise ratio of the detection system.
A device for continuously monitoring the biological activity in a specimen is described by Eden in U.S. Pat. No. 5,366,873. It describes a device and method for detecting microbial growth from a sample substance. The device includes a container which is at least partially transparent and fluid disposed in the container for cultivating microorganisms therein. An indicator substance is disposed in the fluid layer for undergoing transformation in the presence of microorganism growth. A second layer, composed of semi fluid substance, indicators and other substances, such as growth media, is disposed in the container. The substances within the semi-fluid phase are in equilibrium with the substances in the fluid layer and provide a barrier to solid substances introduced into the fluid layer while providing a zone within which changes in the indicator substance, due to microbial growth, can be detected. In practice, the indicator substance has been dyes that are affected by the PH variations in the fluid layer.